The present invention relates to a method for controlling a rate of shrinkage caused upon firing a ceramic green body. More particularly, the present invention relates to a method for controlling a rate of firing shrinkage of a ceramic green body by adjusting properties of a raw material made of ceramic powders.
In producing a ceramic product, controlling a rate of firing shrinkage of a ceramic green body has conventionally been important for improving size precision and shape precision and avoiding a shape defect or the like of a final ceramic product.
Particularly, multilayered ceramic substrates have recently been used upon producing electronic parts such as IC and condenser. Since such electronic parts are required to be put under a precise size and shape control, it is very important to precisely control a firing shrinkage of a green sheet which constitutes such a multilayered ceramic substrate.
As methods for controlling firing shrinkage of a green sheet, some methods have been known. In one method, a mixing time is adjusted so as to adjust a particle size of ceramic powders when a slurry is prepared by mixing ceramic powders, a binder, a plasticizer, a dispersant, a solvent, and the like. Another method is that an amount of a binder is adjusted. There is still another method in which a highest temperature, a retention time, an atomspheric composition, and a gas flow amount are adjusted. Further, Japanese Patent Publication 5-82339 discloses yet another method in which ionic impurities are added to a slurry of a ceramic oxide.
However, these conventional methods have some problems. The first method in which a mixing time is adjusted has a problem that a homogenous slurry cannot be obtained because of insufficient mixing of ceramic powders, a binder, and the like when the mixing time is too short. On the other hand, when the mixing time too long, a flowability of a slurry deteriorates, and a slurry sometimes becomes pudding-like. Further, since a time of mixing contributes less to the control of firing shrinkage as the time of mixing becomes longer, an intended effect cannot be obtained even if the materials are mixed for a long time.
In the method in which an amount of a binder is adjusted, strength of a green sheet decreases and handling convenience deteriorates when the amount of a binder is too small. When the amount is too large, binder can hardly be removed, which makes not only a problem that a fired body is prone to have a shape defect such as bulging on the fired body, but also a problem that the fired body is hardly densified.
Further, in the method in which ionic impurities are added, besides the decrease of strength of a fired body, electrical characteristics such as electric conductivity, permittivity, dielectric loss, etc., are changed.
In the method in which firing conditions are adjusted, a fired body is hardly densified. Besides, strength of a fired body decreases because crystalline particles grow, and properties of the fired body are prone to be changed.
The present invention was made in consideration of such conventional problems.
The present inventors have earnestly studied and found that a method for controlling a firing shrinkage with a great preciseness can be provided by adjusting a spherical diameter of ceramic powder, heating the ceramic powder at a specific temperature, and then molding and firing the ceramic powder, which led to a completion of the invention disclosed in U.S. patent application Ser. No. 538,413 filed Oct. 2, 1995, now U.S. Pat. No. 5,753,160 issued May 19, 1998.
However, such a case that a powder used for molding the composition of a ceramic green body coincides with that of a powder used to obtain a correlation for controlling firing shrinkage of the ceramic green body is not necessarily frequent during production. Particularly, modification of a property of the fired body according to a request from a user requires to mix of other components depending on the property, which makes the work for obtaining the correlation very complex. Thus, the method had disadvantages of high production cost, etc.
Though a correlation present between materials having the same composition was tentatively applied to such a case, the obtained fired body showed that a firing shrinkage of the green body not always be controlled as desired.